A Diesel engine is conventionally equipped with an exhaust gas aftertreatment system that includes an exhaust gas pipe, for directing exhaust gases from the engine to the environment, and a plurality of aftertreatment devices located in the exhaust gas pipe, for reducing and/or removing pollutants from the exhaust gas before discharging it in the environment. In greater detail, a conventional aftertreatment system generally includes a Diesel Oxidation Catalyst (DOC), for oxidizing hydrocarbon (HC) and carbon monoxides (CO) into carbon dioxide (CO2) and water (H2O), and a Diesel Particulate Filter (DPF), located in the exhaust gas pipe downstream the DOC, for removing diesel particulate matter or soot from the exhaust gas. In order to reduce NOx emissions, most aftertreatment systems further include a Selective Reduction Catalyst (SCR), which is located in the exhaust gas pipe downstream of the DPF.
The SCR is a catalytic device in which the nitrogen oxides (NOx) contained in the exhaust gas are reduced into diatomic nitrogen (N2) and water (H2O), with the aid of a gaseous reducing agent, typically ammonia (NH3), that is absorbed inside the catalyst. The ammonia is obtained through thermo-hydrolysis of a Diesel Exhaust Fluid (DEF), typically urea (CH4N2O) that is injected into the exhaust gas pipe through a dedicated injector located between the DPF and the SCR.
More recently, Selective Catalytic Reduction wash-coated particulate filters (also referred to as SDPFs) have been introduced in the aftertreatment system architecture. A SDPF is an SCR (Selective Catalytic Reduction) catalyst coated on a porous DPF (Diesel Particular Filter).
In the aftertreatment system, a NOx sensor is provided to measure the NOx concentration values in the exhaust gas and the NOx values measured are sent to an electronic control unit (ECU), in order to calculate the quantity of DEF (Diesel Exhaust Fluid), typically urea, to be injected in the exhaust gas pipe for achieving an adequate NOx reduction inside the SCR portion of the SDPF.
It is also known in the art to estimate the quantity of soot accumulated in the DPF portion of the SDPF by means of a measure of a differential pressure between the inlet and the outlet of the DPF, for example employing respective pressure sensors, and then, using a model based on the physical characteristics of the DPF, referred in the following disclosure as physical soot model, the ECU calculates an estimated value of the soot quantity accumulated in the DPF.
This known physical soot model however is based on the hypothesis of a strict correlation between the pressure drop through the DPF and the soot quantity trapped thereon: however, this correlation is altered by the so-called CRT effect (Continuously Regenerating Trap), an apparent or effective spontaneous regeneration, which causes a pressure drop reduction across the DPF and eventually soot burning into the DPF. The occurrence of this phenomenon is basically dependent on the local temperature and it is mainly influenced by NO2 levels across DPF.
In order to take into account this effect, a known physical soot model has been employed in the prior art. However, it has been observed that the known physical soot model is not able to correctly estimate the CRT effect for an SDPF architecture, since the continuous regeneration phenomenon is also influenced by the urea injection that occurs upstream of the SDPF. More particularly, in a SDPF architecture, in addition to the temperature, the soot estimation is also affected by differences in the NO2/NOx ratio, a ratio that is modified by urea injection.